1. Field of the Invention
The present invention relates to an emission control system that has at least one catalyst disposed in an exhaust passage for purifying exhaust gas, specifically the system speeds catalyst warm-up and diagnoses the catalyst warm-up speeding function.
2. Related Art
As an apparatus for purifying exhaust gas discharged by an internal combustion engine (referred to hereafter simply as an engine), there has been known an emission control system having a three way catalyst converter (referred to hereafter simply as a converter). When the catalyst is warmed up to an activation temperature, the catalyst exhibits an exhaust gas purifying effect. Thus, the exhaust gas purification rate of the catalyst is low right after a start of the engine. In order to obtain a high exhaust gas purification rate at an early time after a start of the engine, there has been provided control to speed the operation to warm up the catalyst.
Japanese patent JP-P2001-132438A discloses control to speed the operation to warm up a catalyst and a diagnosis of a function to speed the operation to warm up the catalyst. In accordance with the control to speed the operation to warm up the catalyst, an intake air volume is increased and an ignition timing is delayed to increase the temperature of exhaust gas. The diagnosis determines whether the control to speed the operation to warm up the catalyst is functioning normally or abnormally on the basis of an estimated temperature of the catalyst and an actually detected temperature of the catalyst. Estimation of the temperature of the catalyst is based on a cumulative value of the intake air volume or the lapse of time since a start of the engine. However, an accurate diagnosis is hard to carry out. In addition, a sensor for detecting an actual temperature of the catalyst is also required.
Japanese patent JP-P2001-132526A also discloses control to speed the operation to warm up a catalyst and a diagnosis of a function to speed the operation to warm up the catalyst as well. The diagnosis disclosed here determines whether the control to speed the operation to warm up the catalyst is functioning normally or abnormally on the basis of at least one of an engine revolution speed and an ignition timing. By basing the determination on only one of the engine revolution speed and the ignition timing, however, the determination may produce an incorrect result. With only one of the engine revolution speed and the ignition timing, for example, the amount of heat added to the catalyst cannot be estimated precisely.
In addition, neither Japanese patent JP-P2001-132438A nor Japanese patent JP-P2001-132526A discloses backup processing, which is to be carried out in case the control to speed the operation to warm up the catalyst is not functioning normally. Thus, even if an abnormality is determined to exist, the engine is operated in an abnormal state caused by a late operation to warm up the catalyst.
Furthermore, the control techniques to speed the operation to warm up the catalyst and the diagnoses of a function to speed the operation to warm up the catalyst, which are disclosed in Japanese patent JP-P2001-132438A and Japanese patent JP-P2001-132526A, have a problem of an inability to reveal a cause of a detected abnormality.
Moreover, from another aspect, the disclosed art is considered to raise a problem that the catalyst may return back to an inactive state after being activated in some cases. If an idling operation or a fuel-cut operation has been continuing for a long time at a low ambient temperature, for example, the catalyst may return back to an inactive state in some cases.